The present invention relates to a digital encoder and more particularly to a device for providing digitally encoded information of the altitude of an aircraft vehicle.
Various prior art devices have sought to measure the altitude of an aircraft using encoding techniques. One type of device is an optical encoder which uses a disc having a number of successive transparent and opaque portions representing bits of digital codewords. The discs are mounted on a shaft and rotated mechanically. Since these devices are mechanical, they are expensive and do not provide high resolution of altitude.
Electronic devices have sought to measure altitude by providing a curve which approximates altitude vs. barometric pressure. Barometric pressure is related to altitude on a nonlinear scale which is approximately exponential. Such electronic devices have sought to provide the pressure vs. altitude curve by successive approximations of the non-linear curve using small discrete linear portions. This technique requires a large number of approximations which increases the number of circuits, and hence, the complexity of the device. In order to use this technique with any degree of accuracy, it is also necessary to use components which are exceedingly stable with time and temperature. Thus, this technique is costly if employed to provide very accurate altitude measurement.
In another approach, altitude has been measured by using a pressure transducer to measure the barometric pressure in the environment surrounding the aircraft vehicle and convert it to a digital word which is fed to a read only memory (ROM). The ROM operates in a "table lookup" fashion to provide an output in response to input information. This technique is subject to errors in altitude measurements over a large range of altitudes because of non-linear altitude vs. pressure relationship.